The present invention relates to microwave devices comprising a number of parallel-plate resonators allowing selection of modes. The invention also relates to a method of suppressing undesired modes in a microwave device.
It is often desirable to be able to select the modes of microwave devices such as microwave resonators and filters. WO 98/32187 shows the use of aperiodic gratings for mode conversion/selection. However, the grating structures/surfaces are of complex shape and long. These devices furthermore suffer the drawback of being complicated and costly to fabricate and it is also difficult to obtain a mode selectivity which is as accurate as would be desired. Still further they can not be used for thin film resonators for which the thickness is less then xcexg/2, xcexg being the wavelengths of the microwave signal in the resonator. In several implementations it is however desirable to be able to use such resonators. Still further, the size of the resonators is changed when structures as in WO 98/32187 are used.
The Swedish patent application SE 9502137-4, which is the counterpart of allowed U.S. application Ser. No. 08/989,166, filed Dec. 11, 1997, discloses parallel-plate resonators, specially with superconducting plates for low-loss narrow-band filter applications. In xe2x80x9cLower Order Modes of YBCO/STO/YBCO Circular Disc Resonatorsxe2x80x9d, IEEE Transactions on Microwave Theory and Technics, Vol. 44 (10), pp. 1738-1741, 1996, it is shown that in electrical thin resonators (the thickness being smaller than xcexg/2), the higher order TM modes, so called whispering gallery modes, have higher quality factors. It would thus be desirable to utilize these modes in low loss narrow band filter applications. It is however a drawback related to using higher order modes since due to the resonant frequencies of these modes being very close to each other, the rejection bands of for example filters have parasitic undesirable transmission poles, i.e. in other words they are not free of spurious components. SE 9701450-0 xe2x80x9cArrangement and method relating to microwave devicesxe2x80x9d suggests one way to overcome this problem through the use of special mode selective coupling loops. However, such a device is comparatively bulky and most suitable for input/output coupling of resonators in multiresonator filters. Furthermore, since the coupling loops are quite bulky for certain applications, the parasitic modes will not be sufficiently suppressed. Still further such coupling loops are not possible to use in the resonators away from the input/output ports of for example filters.
U.S. Pat. No. 5,710,105 shows high power, high temperature superconductor filters having TM0i0 mode circular shaped high temperature superconductor planar resonators. To suppress interfering non TM0i0 modes, radially directed slots are provided which are positioned parallel to the current of the desired operating mode and perpendicular to the current of an undesired mode. However, these slots are centered at the radius of the disk. They do not cut the maxima. Moreover such slots will affect the useful modes. Thus this device will not work as efficiently as needed. Moreover, this document merely contemplates the TM0i0-modes as attractive for selection.
Therefore microwave devices, particularly microwave resonators and filters, are needed which are mode selective, particularly with a precise mode selectivity. Particularly, devices are needed wherein means enabling mode selectivity are provided which are suitable for use for input/output coupling as well as away from input/output ports of resonators of filters. Particularly a device is needed which is small and for example comprises thin resonators, particularly having a thickness smaller than xcexg/2, xcexg being the microwave wavelength in the resonator. Still further a device is needed through which it is possible to use higher order TM modes in low loss narrow band filter applications. Particularly a device is needed through which higher order modes having close resonant frequencies can be used and through which parasitic and undesirable transmission poles can be avoided. Particularly a device is needed through which any standard thin film fabrication technology can be used and through which mode selectivity is enabled without changing the size of the resonators. Still further a device is needed which generally is inexpensive and easy to fabricate and through which the use of higher order TM modes is enabled without problems being caused by the close resonant frequencies of such modes. A method of suppressing undesired modes in such devices is also needed. A device and a method respectively is also needed which is more efficient in suppressing undesired modes than hitherto known devices at the same time as the effect of the suppression of undesired modes on the desired modes is minimized. Further yet a device and a method respectively is needed through which any mode can be selected or suppressed.
Therefore a microwave device is provided which particularly comprises a number of parallel-plate resonators. Each parallel-plate resonator comprises at least one dielectric substrate with first and second conducting (superconducting) plates arranged on either side of said dielectric substrate. The field (the field produced by coupling arrangement or similar, e.g. discussed in the applications by the same applicant which are incorporated herein by reference above) generates currents in both of the plates of the parallel-plate resonator or resonators (the resonator is thin). At least one of the first and second plates of each of a number of the parallel-plate resonators is patterned or formed in such a way, or comprises current interrupting means, that the current lines of at least one undesired mode are interrupted at their maxima (where the current lines have a maximum) to suppress the undesired mode or modes, thus providing for selectivity. The current interrupting means may be provided in a number of different ways, as actual means or as a particular pattern in, or forming of, the resonators. According to one embodiment the current interrupting means are constituted of cuts in at least one resonator plate of one or more parallel-plate resonators. Particularly the resonator plates comprise metal and the current interrupting means consists of metal being removed except for along the current lines of the desired modes which, in other words, means that the parallel-plate resonator is patterned or formed in such a way.
In an alternative embodiment, the resonator plates comprising metal strips, are the current interrupting means formed by resistive strips arranged along the current lines of the undesired modes, thus replacing the metal strips. This is particularly convenient if the device comprises a number of electrically tunable resonators requiring whole resonator plates, i.e. resonator plates which should not contain any cuts or similar. Also in other implementations requiring xe2x80x9cwholexe2x80x9d resonator plates this implementation consisting of replacing metal strips through resistive strips, is appropriate.
For parallel-plate resonators, or devices built of or including parallel-plate resonators, the current interrupting means may either be provided on one only of the resonator plates of a respective parallel-plate resonator or current interrupting means may be provided on both plates. In a particular implementation the device comprises one or more circular parallel-plate resonators.
Particularly one or more modes are suppressed. In some embodiments the current interrupting means, i.e. the cuts, resistive films or removed metal parts, are arranged to interrupt the current lines of for example one or more of the TM210, TM310 and TM410 modes respectively. Then a number of current interrupting means are arranged which are directed substantially towards the center of the circular parallel-plate resonator. The current interrupting means are so formed that they have a larger width at the edge of the disc whereas the width is substantially zero, or zero, at the midpoint or at a distance from the midpoint thus promoting the desired modes, or not affecting the desired modes.
In one embodiment the current interrupting means are arranged at a distance from the periphery and along at least a part (exceeding 180xc2x0) in the form of a stripe or similar of at least one plate to suppress the TM020 mode. In one embodiment current interrupting means are arranged to suppress the TM110 mode and the current interrupting means are then arranged along a diameter of at least one of the resonator plates and forming substantially 90xc2x0 of the current lines to suppress the mode.
In alternative embodiments a parallel-plate resonator is rectangular, square-shaped or of any appropriate regular or irregular shape.
In a number of alternative embodiments current interrupting means are provided for both plates of a parallel-plate resonator. The current interrupting means of each of the plates of a parallel-plate resonator may then be similar and symmetrical. Also in this case a parallel-plate resonator may be circular, square-shaped, rectangular or of any other convenient shape.
In a particular embodiment the device relates to a filter formed of a number of parallel-plate resonators as referred to above. In a particular implementation the filter is a narrow-band filter.
The electric substrate of the resonator may consists of different materials such as alumina (Al2O3), sapphire, quartz, STO etc. The plates may be normal metal plates, superconducting plates or particularly high temperature superconducting. The inventive concept is particularly applicable on devices as disclosed in the Swedish patent application xe2x80x9cTunable Microwave Devicesxe2x80x9d, 9502137-4, which is the counterpart of allowed U.S. application Ser. No. 08/989,166, filed Dec. 11, 1997 and is hereby incorporated herein by reference. The device enabling exact mode selectivity can advantageously be used in wireless communication systems.
A method of suppressing undesired modes in a microwave device which comprises a number of parallel-plate resonators wherein each resonator includes a first and a second plate and wherein a field generates currents in both of said electrode plates is disclosed which comprises the step of interrupting the maximas of the current lines of the undesired modes in at least one of said plates. According to one implementation the method comprises the step of providing cuts/slots to interrupt the current lines of the undesired mode or modes in the maximas in at least one of the plates. In a particular implementation symmetric cuts/slots are provided in both electrode plates.
In an alternative embodiment a method comprises the step of removing electrode plates throughout at least one of the plates except for along the current lines of the desired mode or modes. In still another embodiment a method includesxe2x80x94the step of arranging resistive strips along the current lines of undesired modes as a replacement for existing metal strips of said resonator plate or plates.
According to the invention the cuts/slots/resistive strips/removed material are positioned predominantly at the maximas of the current lines or current distribution of the modes to be suppressed and at the minimas of the current lines (distribution of the desired modes).
The cuts/slots/resistive strips/removed material may in general have a rectangular shape, but preferably their shape is selected based on the current distribution of undesired modes such that they are maximally suppressed while leaving the desired modes to the highest possible extent unaffected. Thus a careful observation of the maximas of current lines of undesired modes is highly important.
Moreover, according to the inventive concept also other modes than the TM020 or particularly TM020, can be selected as desired modes. Such other modes may have a higher Q-factor which make them very attractive for the fabrication of the filters.